A liquid crystal display is being widely used as a most important display device in the multimedia society, including applications ranging from a cellular phone to a computer monitor, a laptop computer and a television set. In a liquid crystal display, many optical films are used so as to enhance display characteristics. Among others, an optical compensation film plays a great role in, for example, improving the contrast or compensating the color tone when the display is viewed from the front or oblique direction.
The liquid crystal display includes many systems such as vertical alignment type (VA-LCD), in-plane switching liquid crystal display (IPS-LCD), super twisted nematic liquid crystal display (STN-LCD), reflective liquid crystal display and transflective liquid crystal display, and an optical compensation film suited for the display is required.
As the conventional optical compensation film, a stretched film of a cellulose-based resin, a polycarbonate, a cyclic polyolefin, etc. is used. In particular, a film composed of a cellulose-based resin, such as triacetyl cellulose film, is being widely used because of its good adhesiveness to a polyvinyl alcohol as a polarizer.
However, the optical compensation film composed of a cellulose-based resin has several problems. For example, although a cellulose-based resin film is processed into an optical compensation film having a retardation value appropriate to various displays by adjusting the stretching conditions, the three-dimensional refractive indices of a film obtained by the uniaxial or biaxial stretching of a cellulose-based resin are ny≥nx>nz and in order to produce an optical compensation film having other three-dimensional refractive indices, e.g., three-dimensional refractive indices of ny>nz>nx or ny=nz>nx, a special stretching method of, for example, adhering a heat-shrinkable film to one surface or both surfaces of the film and heating/stretching the laminate to apply a shrinking force in the thickness direction of the polymer film is required, which makes it difficult to control the refractive index (retardation value) (see, for example, Patent Documents 1 to 3). Here, nx indicates the refractive index in the fast axis direction (the direction having a minimum refractive index) in the film plane, ny indicates the refractive index in the slow axis direction (the direction having a maximum refractive index) in the film plane, and nz indicates the refractive index outside the film plane (thickness direction).
In addition, a cellulose-based resin film is generally produced by a solvent casting method, and the cellulose-based resin film deposited by a casting method has an out-of-plane retardation (Rth) of about 40 nm in the film thickness direction and therefore, raises a problem, such as occurrence of a color shift in an IPS-mode liquid crystal display, etc. Here, the out-of-plane retardation (Rth) is a retardation value represented by the following formula:Rth=[(nx+ny)/2−nz]×d (wherein nx represents the refractive index in the fast axis direction in the film plane, ny represents the refractive index in the slow axis direction in the film plane, nz represents the refractive index outside the film plane, and d represents the film thickness).
A retardation film composed of a fumaric acid ester-based resin has also been proposed (see, for example, Patent Document 4).
However, the three-dimensional indices of a stretched film composed of a fumaric acid ester-based resin are nz>ny>nx and, for example, lamination to another optical compensation film, etc. is needed for obtaining an optical compensation film exhibiting the above-described three-dimensional refractive indices.
As for the optical compensation film exhibiting the above-described three-dimensional refractive indices, a resin composition and an optical compensation film using the same have been proposed (see, for example, Patent Document 5).
Here, the retardation film is generally used also as an antireflection layer of a reflective liquid crystal display device, a touch panel or an organic EL and in these uses, a retardation film giving a larger retardation in the longer wavelength region (hereinafter, referred to as “reverse wavelength dispersion film”) is required, but Patent Document 5 is silent on use as a reverse wavelength dispersion film.
In the case of using a reverse wavelength dispersion film as the antireflection film, the retardation is preferably about ¼ of the measured wavelength λ, and the ratio Re(450)/Re(550) between the retardation at 450 nm and the retardation at 550 nm is preferably close to 0.81. In addition, with the consideration of thinning of the display device, the reverse wavelength dispersion film is also required to be thin. To meet these requirements, various retardation films are being developed.
As such a retardation film, a retardation plate having a reverse wavelength dispersion property, obtained by the blending of a polymer having positive intrinsic birefringence and a polymer having negative intrinsic birefringence, is disclosed (see, for example, Patent Document 6). In this patent document, a norbornene-based polymer as a polymer having positive intrinsic birefringence, a styrene-maleic anhydride copolymer as a polymer having negative intrinsic birefringence, and a composition obtained by blending these polymers are disclosed, but in the retardation plate using the composition, Re and Nz do not satisfy the relationship preferred as the retardation characteristics of a retardation film.